Psychrometer and method

ABSTRACT

Apparatus and method for use in the measurement of the water or solvent potential of selected samples. The apparatus utilizes a psychrometer principle, which, broadly speaking, relies upon comparison between wet and dry bulb temperatures in a controlled system for obtaining desired measurements. A thermocouple utilized in making psychrometer measurements is concealed within a small psychrometer chamber and is exposed to the sample during testing, being sealed from surrounding environment. The psychrometer chamber is formed by a sample holder and metal heat sink components respectively disposed above and below the sample holder and serve to achieve rapid thermal equilibrium. The interior water potential measuring apparatus including the thermocouple is also thermally insulated to minimize ambient temperature effects. Samples to be analyzed are placed within a cup of a disc-shaped sample holder resting in a slide which can be displaced from side-to-side to position the sample and sample holder medially within the measuring apparatus at the psychrometer chamber. The tightening of a screw insures that the psychrometer chamber is sealed from the surrounding environment, following which vapor pressure equilibration and temperature equilibrium result. A second thermocouple may be provided giving a temperature reading for use in analysis of the electrical output of the psychrometer thermocouple. The output of the psychrometer thermocouple is presented in usable information form at a microvoltmeter which comprises a readout device for the sampling psychrometer.

United States Patent [19] Campbell 51 June 19, 1973 PSYCHROMETER ANDMETHOD [75] Inventor: Eric C. Campbell, Providence, Utah [73] Assignee:Wescor, Inc., Logan, Utah [22] Filed: Feb. 9, 1971 [21] Appl. No.:113,846

[52] US. Cl.

OTHER PUBLICATIONS Zollinger et al., A Comparison of Water-PotentialMeasurements Made Using Two Types of Thermocouple Psychrometer in SoilScience, Vol. 102, No. 4, 1966, pg. 23l-239.

Primary Examiner-Herbert Goldstein Attorney-Lynn G Foster [57] ABSTRACTApparatus and method for use in the measurement of the water or solventpotential of selected samples. The apparatus utilizes a psychrometerprinciple, which,

broadly speaking, relies upon comparison between wet and dry bulbtemperatures in a controlled system for obtaining desired measurements.A thermocouple utilized in making psychrometer measurements is concealedwithin a small psychrometer chamber and is exposed to the sample duringtesting, being sealed from surrounding environment. The psychrometerchamber is formed by a sample holder and metal heat sink componentsrespectively disposed above and below the sample holder and serve toachieve rapid thermal equilibrium. The interior water potentialmeasuring apparatus including the thermocouple is also thermallyinsulated to minimize ambient temperature effects. Samples to beanalyzed are placed within a cup of a discshaped sample holder restingin a slide which can be displaced from side-to-side to position thesample and sample holder medially within the measuring apparatus at thepsychrometer chamber. The tightening of a screw insures that thepsychrometer chamber is sealed from the surrounding environment,following which vapor pressure equilibration and temperature equilibriumresult. A second thermocouple may be provided giving a temperaturereading for use in analysis of the electrical output of the psychrometerthermocouple. The output of the psychrometer thermocouple is presentedin usable information form at a microvoltmeter which comprises a readoutdevice for the sampling psychrometer.

9 Claims, 10 Drawing Figures PAIENIEDJUM 9 ms SllEEI10f3 m N UK 2amowoncm or B Q o tloaomgw INVENTOR. ERIC c. CAMPBELL ATTORNEY PAIENIEU3.739.629

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1 PSYCI-IROMETER AND METHOD BACKGROUND 1. Field of the Invention Thepresent invention relates generally to the art of measuring water orsolvent potential and more particulary to a novel Peltier thermocouplepsychrometer apparatus and related methods for both field and laboratoryuse, the apparatus being used in conjunction with a read out device.

2. Prior Art Psychrometers of the past have generally been of two types,wet loop psychrometers and Peltier thermocouple psychrometers, each typebeing relatively large and expensive and requiring precise control forsatisfactory results. Where prior art psychrometers have been used underlaboratory conditions, a carefully controlled temperature bath has beenrequired, resulting in excessive time delay in reaching temperature andvapor pressure equilibrium in order to produce accurate readings. I amaware of the following prior art publications:

ZOLLINGER, W.C. et al. 1966. A Comparison of Water-PotentialMeasurements Made Using Two Types of Thermocouple Psychrometer. SoilScience Vol. 102, No. 4, pp. 23l-239.

CAMPBELL, G.S. et al. 1966. Sample Changer for ThermocouplePsychrometers: Construction and Some Applications. Agronomy Journal Vol.58, pp. 315-318.

CAMPBELL, G.S. et al. A Welding Technique for Peltier ThermocouplePsychrometers. pp. l-6.

CALISSENDORFF, C. et al. 1970. Construction And Calibration Of An InSitu Leaf Psychrometer With Small Temperature Sensitivity. W-67 RegionalResearch Report. pp. l-6.

RAW'LINS, S.L. et al. 1967. Psychrometric Measurement of Soil WaterPotential Without Precise Temperature Control. Soil Science Society ofAmerica Proceedings Vol. 31, No. 3 pp. 297-300.

RAWLINS, S.L. et al. 1968. In Situ Measurement of Soil and Plant LeafWater Potential. Soil Science Society of America Proceedings Vol. 32,pp. 468-470.

DALTON, F.N. et al. 1968. Design Criteria For Peltier-EffectThermocouple Psychrometers. Soil Science Vol. 105, No. I pp. 12-17.

RAWLINS, S.L.' 1966. Theory For Thermocouple Psychrometers Used ToMeasure Water Potential In Soil And Plant Samples. AgriculturalMeterology Vol. 3 pp. 293-310.

WIEBE, Herman E. et al. 1968 or later. Measurement of Water PotentialGradients in Trees. pp. 4-14.

SUMMARY AND OBJECTS E THE PRESENT I INVENTION eter or sample chamber,which is located between two heat sinks, without exposing the interiorof the psychrometer including the sample chamber to surroundingenvironmental conditions. An 0" ring seal prevents such exposure and isflattened by force-applying structure so that the sample holder withinthe sample chamber and the two heat sinks becomes contiguous wherebyuniform temperature conditions prevail inside the psychrometer. Theexterior bottom surface of the psychrometer is recessed to minimize theimpact of external temperature upon the psychrometer, while the sampleholder within the sample chamber and two heat sinks are thermallyinsulated.

Accordingly, it is a primary object of the present invention to providenovel Peltier thermocouple psychrometers and related methods.

Another important object of the present invention is the provision formeasuring water or solvent potential in the laboratory or in the fieldby comparison of wet and dry bulb temperatures in a small test region ofthe pyschrometer.

It is a further significant object of the present invention to providean improved psychrometer which utilizes a slide and spaced sampleholders carried by the slide, and accommodates serial introduction ofthe sample holders into the sample chamber without exposing the interiorof the psychrometer to surrounding environmental conditions. t

It is a further paramount object of the present invention to provide anovel psychrometer which serially comprises two heat sinks with a slideinterposed between for introducing sample holders with samples thereininto a sample chamber adjacent one heat sink.

A further important object comprises the provision of novel sealstructure for preventing exposure of the sample chamber of apsychrometer during repeated testing of a plurality of samples.

Another significant object of this invention comprises the provision offorce-applying structure to bring a sample holder and two heat sinks,one on each side of the holder, into contiguous relation to form asingle thermal mass during testing.

Another principal object of the present invention is the provision of animproved psychrometer which is recessed at its exterior bottom surfaceto minimize environmental temperature effects upon. the psychrometer.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of apresently preferred apparatus according to this invention;

FIG. 2 is an enlarged cross section taken along the line 2-2 of FIG. 1;l l

FIG. 3 is an enlarged cross section taken along the line 3-3 of FIG. 1;

FIG. 4 is an enlarged fragmentary cross section illustrating thepsychrometer chamber, the heat sink assemblies and the sample holder ofthe apparatus of FIG. -1;

FIG. 5 is an enlarged view taken along the line 5-5 of FIG. 4,illustrating the upper heat sink assembly and the psychrometerthermocouple;

FIG. 6 is a cross section taken along line 6-6 of FIG. 2;

3 FIG. 7 is a cross section taken along line 77 of FIG.

FIG. 8 is a diagram illustrating a presently preferred circuit of thepresent invention;

FIG. 9 is one graphic representation of actual and meter-indicatedtemperature depressions across the psychrometer thermocouple; and

FIG. 10 is typical calibration curve which can be used to convert ameter-indicated temperature depression to water or solvent potential fora given sample.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT General The samplechamber psychrometer, illustrated in the Figures and generallydesignated 10, comprises apparatus and accommodates methods relating tothe use of the psychrometer principle in the measurement of the water orsolvent potential of small samples, which may comprise aqueous ornon'aqueous liquid, or any substance containing liquid, such as leafspecimens, soil specimens, medical solutions, any plant or animaltissue, etc., either in the laboratory or in the field, without the useof a constant temperature bath. Measurements are made by thepsychrometer 10 and a read out is produced, as, for example, by avoltmeter 84 (FIG. 8). Samples are each placed in a sample holder 46carried by a slide and are successively introduced within a smallpsychrometer chamber 92 where the water or solvent potential is latermeasured. Heat sinks 86 and 78 are situated respectively above and belowthe holder 46 and together become a single thermal mass during testingproviding for rapid thermal equilibrium. The psychrometer chamber 92 isentirely thermally insulated by the cylinder assembly 12 to minimizeambient temperature variations of the sample, the holder, and the heatsinks.

The apparatus 10 provides for the sealing from surrounding environmentof a concealed psychrometer thermocouple T1 (FIGS. 4, 5 and 8) and asample and holder 46 within the psychrometer chamber 92. Thethermocouple T1 is a chromelconstantan thermocouple enclosed within thepsychrometer chamber 92. The apparatus It) utilizes the mentionedpsychrometer principle, which pertains to comparison between wet and drybulb temperatures in a gaseous atmosphere of solvent or water as ameasurement of the relative partial pressure of the gas form of thesolvent or water on the wet bulb or thermocouple junction J (FIG. 8).

The present invention utilizes the Peltier effect. The Peltier effectmay be defined as the absorption or liberation of heat at thethermocouple junction J caused when a controlled current flows acrossthe junction J, which consists of dissimilar metals, the direction ofthe current flow controlling whether absorption or liberation ofheatresults and is proportional to the current flow.

The water or solvent potential, depending upon whether the liquid underconsideration is aqueous or non-aqueous, may be defined as the amount ofenergy per unit mass of solvent required to remove pure solvent from thesample in question, and is usually measured in joules of energy perkilogram of water or solvent, as the case may be.

After allowing time for vapor pressure equilibration and temperatureequilibrium within the chamber 92, a controlled current is caused topass through the thermocouple T1 from the battery B1 in a direction suchthat the thermocouple T1 is cooled by absorption of heat, causingmoisture to condense on the junction J. After a cooling period,switching of mechanically linked switches S1 and S2 is performed to haltthe mentioned current flow through the thermocouple T1. The mentionedswitching also connects the outputs of the thermocouple T1 at wires W1and W2 through the amplifier 85 to a voltmeter 84, which measures theelectromotive force output from the junction. The electromotive forceoutput, in voltage units, is proportional to the rate of evaporation atthe thermocouple junction J. The rate of evaporation is a function ofthe water or solvent potential of the sample and can be determinedaccording to available calibration curves, which will be discussed ingreater detail hereinafter.

Structure The osmometer or psychrometer 10 is made up of a cylindricalassembly 12, preferably of nylon material, comprised of threecylindrical wafers, i.e. an upper wafer 14, a center wafer 16, and alower wafer 18. A slide 20, also preferably of nylon, is receivedthrough a slot or notch 22 in the top surface of the lower wafer 18immediately vertically below and adjacent the lower edge or face 24 ofthe middle wafer 16. A nylon or like screw 38, with a knurled top, isthreadedly received at threads 62 of the upper wafer 14. The cylindricalassembly l2 thermally insulates the interior of the psychrometer.

The three wafers 14, 16 and 18 are connected together by two socket headscrews 30 (FIG. 3) which are respectively recessed in a counterbore 32in the bottom of the lower wafer 18 and extend vertically upwardscrewing into an interiorly threaded insert or retainer 26. Each insert26 is in turn recessed in a counterbore 28 in the top of the upper wafer14 and extends downward therefrom through aligned bores 29, 66 and 31 ofwafers 14, 16 and 18, respectively. The upper wafer 14 is alsoindependently connected to the middle wafer 16 by two socket head screws68, each of which is recessed into a counterbore in the bottom of themiddle wafer 16 and engages the threads of a bore 64 in the upper wafer14. The threaded part of each screw 68 passes loosely through a bore 74in the middle wafer 16.

The upper wafer 14 has a large, centrally threaded bore 60 whichreceives the threaded portion 62 of the nylon screw 38. The lower wafer18 is constructed with a central recess 39 adjacent its lower edge 36avoiding a large area of contact with the supporting surface to minimizeheat conduction. The slide 20, which is slideably received betweeen thelower edge 24 of the middle wafer 16 and the slot 22 in the top surface33 of the lower wafer 18, can be manually moved from side to sidethrough the slot 22, the limits being determined by the stop pins 56 and58 respectively carried by the slide 20 at opposite ends thereof/Theslide 20 is shown as having two apertures 44, each comprising a lowerlip 42 so as to receive and contain a sample holder 46 therein.Preferably, each sample holder 46 is of brass material coated with acorrosion resistance material,

such as stainless steel. If desired, the slide can be of greater lengthand can comprise several more apertures for receiving sample holders.When the slide 20 is in one extreme position, one stop 56 or 58 isagainst the cylindrical assembly 12 and the sample holder 46 in oneaperture 44 is positioned in the center of the cylindrical assembly 12as shown in FIG. 2. With the sample holder 46 in the position of FIG. 2,the'sample holder 46 is directly above a heat sink 78 preferably ofbrass which rests in a blind bore 76 in the lower wafer 18. The lowersurface 52 of the sample holder 46 is contiguous with the upper surface82 of the heat sink 78. P- sitioned vertically above the sample holder46 is a generally cylindrical heat sink and thermocouple assembly 86,which is inserted into a central bore 72 of the middle wafer 16. Theupper surface 88 of the upper heat sink and thermocouple assembly 86 isadjacent to the lower surface 40 of the nylon screw 38. The lowersurface 90 of the heat sink assembly 86 is adjacent the upper surface 50of the sample holder 46.

As mentioned, the upper heat sink and thermocouple assembly 86 isreceived within the bore 72 of the center wafer 16 in verticallyslideable relation, being limited in its vertical movement by a guidepin 102 (FIGS. 2 and 4) which secured in press-fit relation in atransverse bore 104 in the center wafer 16 and extends transversely intothe bore 72 where it fits into a longitudinal slot 100 in the outsidecasing, which is preferably brass, of the heat sink and thermocoupleassembly 86. Thus, the heat sink assembly 86 is retained within the bore72 between the pin 102 and the sample holder 46.

The lower portion 96 (FIGS. 4 and 5) of the upper heat sink andthermocouple assembly 86 is constructed with a very shallow recessedcenter cavity 91 in which the psychrometer thermocouple T1 is mounted.The chromel and constantan leads or wires W8 andW9, usually havingdiameters of about 0.001 of an inch, of the thermocouple T1 extenddownwardly into the cavity 91 through a block of material 114 in thecenter bore 108 of the heat sink and thermocouple assembly 86 and arejoined together, as by soldering or Welding, at junction J, normally ofspherical configuration and about seven times the diameter of the wiresW8 and W9. Thermocouple wires W8 and W9 are respectively electricallyjoined to wires W1 and W2, preferably of copper material. Thethermocouple T1 is, therefore, mounted within the center bore 108 by thestructure 114, such as Teflon, press-fit in place, through which thewires W1 and W2 extend and which defines the top of the shallow cavity91.

The leads W1 and W2 extend from the thermocouple Tl upwardly through thestructure 1 14 and are thereafter packaged together within a sheath 126in electrically insulated relation one to the other. The sheath 126extends from the center aperture 108 (FIG. 6) through a radial slot 110in the heat sink and thermocouple assembly 86. Immediately adjacent theslot 110 at the junction of the heat sink and thermocouple assembly 86and the bore 72 of the middle wafer 16 is a slot 124 in wafer 16 whichcomprises a linear extension of slot 110 through which the sheath 126 ispassed to an annular channel 122 which loops around the heat sink andthermocouple assembly 86, being concentric thereto. The sheath 126 isplaced in the channel 122 and traverses approximately 360 beforeextending through a radial slot 128 which is situated in the wafer 16between the channel 122 and the exterior surface of the wafer 16. Thesheath 126 is placed in channel 122 in order to minimize heat conductionto or from the thermocouple T1.

A second thermocouple T2 (FIG. 6), which may but need not be used,measures the temperature at the interior of the psychrometer and isinserted in contiguous relation with theheat sink and thermocoupleassembly 86 at the slot 110. Cable 130, which comprises two electricallyinsulated wires connects to the temperature thermocouple T2 and isconfined within slots 110 and 124, channel 122 and slot 128 in the samemanner as described in conjunction with sheath 126.

An 0 ring 98 (FIG. 4) is disposed in an annular groove 94 of end portion96 of the assembly 86 and is there retained by a sloping or beveled lip112. As the slide 20 is displaced, the O ring 98 will create a sealagainst the top surfaces of the slide and sample holder to preventexposure of the interior of the psychrometer to surroundingenvironmental conditions. The 0" ring is firmly pressed against the topsurface 50 of the sample holder 46 and creates a seal therewith when onesample holder is aligned with the chamber 91.

As earlier mentioned, the upper surface 88 of the heat sink andthermocouple assembly 86 is vertically below and adjacent the lowersurface 40 of the screw 38. Consequently, the heat sink and thermocoupleassembly 86 is pressed downward when the screw 38 is tightened, causingthe O ring 98 to be flattened. This causes the bottom surface 90 ofassembly 86 to firmly contact the top surface 50 of the sampler holder.At the same time, the bottom surface 52 of the sample holder 46 isfirmly contiguous with the top surface 82 of the heat sink 78.Consequently, the sample chamber 92 is firmly sealed and the heat sinks78 and 86 and the sample holder 46 comprise a single thermal mass.

.If desired, the details of the described embodiment as illustrated inthe figures may be altered without departing from the spirit of thepresent invention. For example, as mentioned, a slide having a pluralityof apertures 44 in excess of two arranged in a continuous row could beutilized. In this way a series of samples can be successively processedinto a psychrometer or sample chamber for testing, by moving such slidein a single direction, a given distance for each sample, withoutexposure of the interior of the psychrometer to outside environmentalconditions.

Electrical Reference is now made to FIG. 8, which illustrates presentlypreferred circuitry for the described circuitry. FIG. 8 schematicallyshows the mentioned chromel-constantan thermocouple T1. The negativepole of a DC. battery B1 is connected by a wire W1 to the chromel leadW8 of the psychrometer thermocouple T1, which chromel lead W8 terminatesat junction J. Wire W2 connects to the constantan lead W9, which alsoterminates at junction J. The lead W1 is also connected to the negativeinput terminal of amplifier 85. Wire W2 is connected to one terminaleach switch S1 and 82, as can be readily observed by inspection of FIG.8.

Wire W4 connects the positive pole of the DC. battery B1 across resistorR1, which serves to control the rate at which current is permitted topass across switch S2 to the thermocouple T1. When switches S2 and S1,

which are mechanically linked together, are in their down positions asillustrated in FIG. 8, current will be passed across resistor R1, alongwire W4, across switch S2, along wires W2 and W9 to the junction J,resulting in cooling of the junction J. This is the cooling mode.

When the linked switches S1 and S2 are in their up position, power fromthe battery B1 is not permitted to reach the thermocouple T1 because ofthe open condition of switch S2, while any electromotive force producedat junction J is communicated along wires W9 and W2 across switch S1 andalong wire W3 to the positive input of the amplifier 85. This is theread mode.

The output of the amplifier 85 is connected by wire W6 to amicrovoltmeter 84, which is grounded by wire W7, the meter providing anappropriate read out of the mentioned electromotive force in microvolts.Of course, other output devices part from voltmeters could be used forthe indicated purpose. For example,

a suitable recorder could be used to provide a permanent record of suchelectromotive force.

As hereinafter more fully explained the meter 84 is initially set atzero by adjusting the zero offset resistor R2, which is the controlinput to the amplifier 85. Operation The operation of the psychrometer land related circuitry will now be explained. One sample holder 46 isplaced in a selected aperture 44 of the slide and a sample is preferablythereafter placed in the sample cavity 48 of the sample holder 46. Thesample holder 46 is then located immediately below the thermocouple T1in sample chamber 92 by appropriate linear manual displacement of theslide 20 in respect to the cylindrical assembly 12. The chamber 91 mustbe clean for proper operation and testing. The screw 38 is thentightened into the upper wafer 14 displacing the upper heat sink andthermocouple assembly 86 downwardly, causing the 0" ring 98 to beflattened while maintaining its seal against the upper surface 50 of thesample holder 46 and causing the two heat-sinks to be firmly contiguouswith the top and bottom respectively of the sample holder so that onethermal mass is formed as previously mentioned. With the switches S1 andS2 in their up position, the electromotive force produced at junction Jis communicated to the positive input of amplifier 85. This measurementimmediately upon placing a sample within the chamber 92 is the dry bulbtemperature. This condition is maintained after the sample holder andsample are properly inserted into chamber 92 until temperatureequilibrium and vapor pressure equilibration result in the samplechamber 92. These conditions become known to the operator when the meterreading becomes constant. At this time, the meter 84 is zeroed by use ofthe zero offset control R2 (FIG. 8), which is connected to the amplifierby wire W5. The switches S1 and S2 are placed in their down or coolingmode positions, as illustrated in FIG. 8. This opens the circuit betweenthe thermocouple T1 along wires W2 and W3 to the amplifier and causes acontrolled current to flow from constantan wire W9 to chromel wire W8across junction J. This causes the junction J to cool, resulting in thecondensation of water or solvent on the surface of the junction J.

Later, the mentioned current flow through the thermocouple T1 isdiscontinued when the switches S1 and S2 are placed in their uppositions as viewed in FIG. 8. Switch S2 is then open, breaking thecircuit between the battery B1 and the junction J, and switch S1 is thenclosed, completing the circuit from the junction J across constantanwire W9 of the thermocouple Tl along wire W2 to the positive input ofthe amplifier at wire W3. Immediately after the switches S1 and S2 havebeen changed, cooling at the junction J stops and the temperature of thethermocouple T1 is different in respect to its original temperature onlyby a factor related directly to the rate at which water or solvent isevaporating from the junction J. The temperature of the thermocouple T1is read by measuring its electromotive force in terms of voltage fromthe amplifier at the meter 84 and is a measure of the rate at whichwater or solvent is evaporating from the surface of the junction J,which is directly related to the osmolality of the sample. In the readmode, the voltage across thermocouple is proportional to the temperaturedifference between the junction J and the average of the junctionsbetween wires W1 and W8 and W2 and W9, respectively. After all the wateror solvent on the surface of the junction J has evaporated it willreturn to its original temperature.

In respect to the graph of FIG. 9, explanation will be made as tovarious meter readings produced during testing of a given sample, theappropriate portion of such readings, which is classified as an accuratemeasure of the electromotive force produced at junction J in the mannerpreviously described, and the relationship of meter-indicated voltage asopposed to actual temperature depression in terms of voltage across thethermocouple. The graph of FIG. 9 is in combination (a) a representationof the actual temperature depression occuring across the thermocouple T1during a complete test cycle and (b) a representation of the temperaturedepression as indicated on the voltmeter 84.

The phantom line, as shown on the graph of FIG. 9, is a representationof the actual temperature depression as a voltage across thethermocouple T1 during a complete test cycle. At thermal equilibrium andvapor pressure equilibration in the psychrometer chamber or samplechamber 92, the actual temperature depression is zero. Thermalequilibrium and vapor pressure equilibration time may vary considerablydepending upon temperature conditions, the nature of the sample inquestion and other factors. For example, equilibration over leaves,normally obtained with a paper punch, and other similar samples may beslow requiring as long as 15 minutes, while soil and larger samplessometimes require on the order of two or three minutes. Equilibration ismuch faster where solutions are being tested, it being normal practiceto saturate a filter paper disc with the solution and thereafter placethe disc in the sample-receiving cup of the sample holder 46.Immediately upon initiation of the cooling cycle, the current flowacross the thermocouple T1 from constantan to chromel produces a rapidcooling which changes the actual temperature depression in terms ofvoltage markedly as shown by the vertical jump in the phantomrepresentation of FIG. 9. The actual temperature depression reaches aceiling when the dew point is reached, i.e. a point in time whenconditions in the psychrometer chamber 92 commence to producecondensation at junction J. This ceiling is identified as Dew Point onthe graph of FIG. 9.

Sometime after the dew point is reached, switches S1 and S2 are shiftedto the up position, as illustrated in FIG. 8, causing a reduction in thetemperature depression across the thermocouple as cooling at junction Jis discontinued and evaporation commences. Consequentially, there is asharp drop in temperature depression in terms of voltage across thethermocouple, which is illustrated in FIG. 9 as that portion of thephantom line immediately above the Spike Point."

During cooling, the switches S1 and S2 are in their down positions, asillustrated in FIG. 8, and the mentioned electromotive force is nolonger communicated to meter 84. As a result a meter readingcorresponding to the amplifier output with the amplifier inputs groundedresults, shown on FIG. 9 as being negative for illustrative purposes andexisting between the 10 and second interval of time. During this periodof time as previously mentioned, the positive pole of the battery is inelectrical communication with the thermocouple TI and cooling andcondensation at the junction J is occurring. While a cooling interval of10 seconds is illustrated in FIG. 9, it is to be appreciated that,depending upon temperature, sample conditions and other factors, shorteror longer intervals may be required.

With continued reference to FIG. 9, following adequate cooling time aspreviously indicated, switches S1 and S2 are again positioned in theirup dispositions as shown in FIG. 8, resulting in a sharp jump in thetemperature depression meter reading, the jump being shown as thevertical dotted line in FIG. 9. At this point, the previously mentionedactual temperature depression across the thermocouple becomes thetemperature depression meter reading and is illustrated in FIG. 9 as theSpike Point.

Thereafter, as illustrated on the portion of the graph seen to the rightof the Spike Point, the actual temperature depression across thethermocouple is also the temperature depression meter reading. After thepeak spike voltage is dissipated, a plateau output results, which isidentified in FIG. 9 as the wet bulb depression or water (solvent)potential reading. During this interval, the operator of thepsychrometer 10 records the essentially constant reading on thevoltmeter, following which the meter reading proceeds to decrease asshown by the remainder of the curved line to the right in FIG. 9,ultimately returning to zero.

Once the wet bulb depression or water (solvent) potential read out hasoccurred for a given sample in the manner indicated above, this value isused in conjunction with a suitable calibration curve (see FIG. 10) toarrive at a corresponding bar value (one bar equals 100joules/kilogram), which constitutes the water or solvent potential ofthe sample under consideration. Suitable calibration curves areformulated by placing a solution of known water or solvent potential inthe psychrometer l0 and obtaining voltage outputs at selectedtemperature for the specific solution. Thus, for a given temperature,the relationship between voltage and water potential becomes a knownempirical relationship which can be graphically produced in the mannershown in FIG. 110 for the particular substance under consideration.Thereafter, by knowing the substance of a smaple being investigated, thetemperature within the sample chamber 92 and the meter reading, thewater potential in bars can be readily ascertained from an appropriatecalibration curve.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is, therefore, to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdesc'ription, and all changes which come within the meaning and range ofequivalency of the claims are therefore to be embraced therein.

I claim:

ll. A psychrometer comprising:

a rigid casing of material having good thermal insulationcharacteristics;

heat sink means disposed within the casing;

a transverse slot disposed through the heat sink means;

a peltier thermocouple disposed immediately adjacent the transverse slotwith the junction of the thermocouple disposed in a chamber which opensdirectly to the transverse slot;

slider means reciprocably situated in the transverse slot for movingsamples into alignment with the chamber and for removing samples fromthe psychrometer;

means sealing the sample of external environmental conditions at leastwhen the sample is situated in alignment with the chamber;

the heat sink means comprising two parts, one on each side of thetransverse slot;

the slider means comprises a slide bar by which at least one sampleholder is carried;

the sample holder and the heat sink means having good thermal conductingproperties;

the sealing means comprises an O ring interposed between one heat sinkpart and one surface of the slider means;

the sealing means further comprising force-applying means forcompressing the two parts of the heat sink means and the sample holdertogether counter to the memory of the O ring, creating a single thermalmass from the three mentioned components.

2. A peltier psychrometer for measuring water and solvent potential of aplurality of samples comprising:

a casing comprising thermal insulating material;

a heat sink mass and peltier thermocouple means situated within thecasing;

slider means extending through the casing and heat sink mass inslideable relation, the junction of the thermocouple being exposed topart of the slider means within the heat sink mass;

means creating and maintaining a sealed relation between the slidermeans and the heat sink mass to isolate the thermocouple junction andthe adjacent portion of the slider means from external environmentalconditions;

slider means comprising a carrier and spaced sample holders situated inalignment along the carrier, whereby the sample holders may be seriallymoved from the environment external to the psychrometer to a positionadjacent the junction for testing purposes without at any time exposingthe junction to external environmental conditions.

3. A psychrometer comprising:

a thermal insulating rigid casing;

thermally conductive heat sinks situated within the casing such that aspace exists therebetween;

the heat sinks being relatively reciprocable within limits one inrespect to the other;

.one heat sink having a moisture-detecting peltier thermocoupleassociated therewith such that the junction of the thermocouple isexposed at the sapce between the heat sinks;

means for introducing a thermally conductive sample holder between theheat sinks so that a sample associated, with the sample holder ispositioned directly adjacent the junction of the thermocouple;

means for forcing the heat sinks into firm contiguous relation with thesample holder thereby creating a single thermal mass to insure rapidthermal equilibrium.

4. A psychrometer as defined in claim 3 further comprising an anularseal interposed between one heat sink and the introduced sample holderto create a sealed relation therebetween.

5. A psychrometer as defined in claim 3 wherein the thermocouplejunction is disposed within a small cavity existing at the one heatsink.

6. A peltier psychrometer assembly comprising:

a casing having thermal insulating properties serially comprising afirst end portion, an intermediate portion and a second end portion,secured together in aligned and contiguous relation by fastener means;

one thermally conductive heat sink being rigidly held in an internalblind bore of one end portion of the casing;

an opening disposed at the interface between the one end portion and theintermediate portion of the casing, the one heat sink being exposed atthe opening;

means displaceably situated in the opening for introducing thermallyconductive sample-confining holders within the casing at the interface;

a second heat sink being reciprocably carried within a central bore ofthe intermediate portion of the casing in exposed relation to theopening;

a peltier thermocouple held centrally within the second heat sink, thejunction of the thermocouple being exposed adjacent the opening;

force-applying means centrally carried by the other end portion of thecasing urging the second heat sink toward the opening, causing the heatsinks to respectively be contiguous with a sample-confining holderdisposed at the interface.

7. A psychrometer as defined in claim 6 further comprising an annularseal interposed between the second heat sink and the sample-confiningholder disposed at the interface.

8. In a method of measuring water and solvent potential of samples, thesteps of:

transversely introducing one sample disposed within one sample carriercarried by a slide mechanism from outside a peltier psychrometer into asample chamber of the psychrometer by displacement of the slidemechanism while maintaining the sample chamber and slide mechanism atthe sample chamber sealed from the environment outside the psychrometer,the junction of the thermocouple of the psychrometer being exposedwithin the sample chamber;

testing the sample by use of the thermocouple to obtain a measure of thewater or solvent potential of the sample; and

transversely introducing a second sample disposed in 0 a second samplecarrier carried by the slide mechanism from outside the psychrometerinto the sample chamber and simultaneously removing the one sample andthe one sample carrier from the sample chamber to a position outside thepsychrometer by displacement of the slide mechanism while maintainingthe sample chamber and slide mechanism at the sample chamber in saidsealed condition.

9. In a method of measuring water and solvent potential of a sample, thesteps of:

placing a sample holder with the sample within a test chamber of apsychrometer where the junction of a peltier thermocouple is exposed tothe sample by moving carrier means which hold a plurality of spacedsamples in respect to the thermocouple;

creating and maintaining a seal against the carrier means between thetest chamber and the external environment enabling successive deliveryof samples to the test chamber by periodic displacement of the carriermeans without exposing the junction to external environmentalconditions;

isolating the sample within the test chamber;

allowing thermal equilibrium and water or solvent pressure equilibrationto occur within the test chamber;

measuring the dry bulb electromotive force at-the junction of thepeltier thermocouple; subjecting thejunction of the thermocouple topeltier cooling, causing condensation of water or solvent at thejunction;

terminating the peltier cooling; measuring the wet bulb electromotiveforce at the junction once constancy is reached; determining thedifference between the dry and wet bulb electromotive forces at thejunction in the isolated gaseous atmosphere of the test chamber as ameasure of the relative partial pressure of the gas form of the water orsolvent.

1. A psychrometer comprising: a rigid casing of material having goodthermal insulation characteristics; heat sink means disposed within thecasing; a transverse slot disposed through the heat sink means; apeltier thermocouple disposed immediately adjacent the transverse slotwith the junction of the thermocouple disposed in a chamber which opensdirectly to the transverse slot; slider means reciprocably situated inthe transverse slot for moving samples into alignment with the chamberand for removing samples from the psychrometer; means sealing the sampleof external environmental conditions at least when the sample issituated in alignment with the chamber; the heat sink means comprisingtwo parts, one on each side of the transverse slot; the slider meanscomprises a slide bar by which at least one sample holder is carried;the sample holder and the heat sink means having good thermal conductingproperties; the sealing means comprises an ''''O'''' ring interposedbetween one heat sink part and one surface of the slider means; thesealing means further comprising force-applying means for compressingthe two parts of the heat sink means and the sample holder togethercounter to the memory of the ''''O'''' ring, creating a single thermalmass from the three mentioned components.
 2. A peltier psychrometer formeasuring water and solvent potential of a plurality of samplescomprising: a casing comprising thermal insulating material; a heat sinkmass and peltier thermocouple means situated within the casing; slidermeans extending through the casing and heat sink mass in slideablerelation, the junction of the thermocouple being exposed to part of theslider means within the heat sink mass; means creating and maintaining asealed relation between the slider means and the heat sink mass toisolate the thermocouple junction and the adjacent portion of the slidermeans from external environmental conditions; slider means comprising acarrier and spaced sample holders situated in alignment along thecarrier, whereby the sample holders may be serially moved from theenvironment external to the psychrometer to a position adjacent thejunction for testing purposes without at any time exposing the junctionto external environmental conditions.
 3. A psychrometer comprising: athermal insulating rigid casing; thermally conductive heat sinkssituated within the casing such that a space exists therebetween; theheat sinks being relatively reciprocable within limits one in respect tothe other; one heat sink having a moisture-detecting peltierthermocouple associated therewith such that the junction of thethermocouple is exposed at the sapce between the heat sinks; means forintroducing a thermally conductive sample holder between the heat sinksso that a sample associated, with the sample holder is positioneddirectly adjacent the junctioN of the thermocouple; means for forcingthe heat sinks into firm contiguous relation with the sample holderthereby creating a single thermal mass to insure rapid thermalequilibrium.
 4. A psychrometer as defined in claim 3 further comprisingan anular seal interposed between one heat sink and the introducedsample holder to create a sealed relation therebetween.
 5. Apsychrometer as defined in claim 3 wherein the thermocouple junction isdisposed within a small cavity existing at the one heat sink.
 6. Apeltier psychrometer assembly comprising: a casing having thermalinsulating properties serially comprising a first end portion, anintermediate portion and a second end portion, secured together inaligned and contiguous relation by fastener means; one thermallyconductive heat sink being rigidly held in an internal blind bore of oneend portion of the casing; an opening disposed at the interface betweenthe one end portion and the intermediate portion of the casing, the oneheat sink being exposed at the opening; means displaceably situated inthe opening for introducing thermally conductive sample-confiningholders within the casing at the interface; a second heat sink beingreciprocably carried within a central bore of the intermediate portionof the casing in exposed relation to the opening; a peltier thermocoupleheld centrally within the second heat sink, the junction of thethermocouple being exposed adjacent the opening; force-applying meanscentrally carried by the other end portion of the casing urging thesecond heat sink toward the opening, causing the heat sinks torespectively be contiguous with a sample-confining holder disposed atthe interface.
 7. A psychrometer as defined in claim 6 furthercomprising an annular seal interposed between the second heat sink andthe sample-confining holder disposed at the interface.
 8. In a method ofmeasuring water and solvent potential of samples, the steps of:transversely introducing one sample disposed within one sample carriercarried by a slide mechanism from outside a peltier psychrometer into asample chamber of the psychrometer by displacement of the slidemechanism while maintaining the sample chamber and slide mechanism atthe sample chamber sealed from the environment outside the psychrometer,the junction of the thermocouple of the psychrometer being exposedwithin the sample chamber; testing the sample by use of the thermocoupleto obtain a measure of the water or solvent potential of the sample; andtransversely introducing a second sample disposed in a second samplecarrier carried by the slide mechanism from outside the psychrometerinto the sample chamber and simultaneously removing the one sample andthe one sample carrier from the sample chamber to a position outside thepsychrometer by displacement of the slide mechanism while maintainingthe sample chamber and slide mechanism at the sample chamber in saidsealed condition.
 9. In a method of measuring water and solventpotential of a sample, the steps of: placing a sample holder with thesample within a test chamber of a psychrometer where the junction of apeltier thermocouple is exposed to the sample by moving carrier meanswhich hold a plurality of spaced samples in respect to the thermocouple;creating and maintaining a seal against the carrier means between thetest chamber and the external environment enabling successive deliveryof samples to the test chamber by periodic displacement of the carriermeans without exposing the junction to external environmentalconditions; isolating the sample within the test chamber; allowingthermal equilibrium and water or solvent pressure equilibration to occurwithin the test chamber; measuring the dry bulb electromotive force atthe junction of the peltier thermocouple; subjecting the junction of thethermocouple to peltier cooling, causing condensation of water orsolvent at the junction; terminating The peltier cooling; measuring thewet bulb electromotive force at the junction once constancy is reached;determining the difference between the dry and wet bulb electromotiveforces at the junction in the isolated gaseous atmosphere of the testchamber as a measure of the relative partial pressure of the gas form ofthe water or solvent.